This means for the next [average] 2 years after you first began your
period, you could go a week, month, 3
months,
5 months, or even a year without your period. One time your period may last for 3 days, another for 7, and then for 5 days.
Usually [average] 2 years after you start your period, it will become regular. You will be able to predict when your period will start, how heavy/light it will be, when it will end, and how long it will last.